> Potential energy, U = mgh. So the energy required to raise 1m^3 of sand 1400m is 1600 x 1400 x 9.8 = 22MJ = 6.1kWh

If you lowered 10 m^3 of sand (61kWh of potential energy), to generate the minimum 100kW power to participate in grid stabilization markets, you'd have to drop that 16000kg of sand for 61kWh/100kW = 0.61hr = 2196 sec. 1400 meters in 2196 seconds is 0.64 m/sec. That seems reasonable, but you'd need a lot of these (so a wide mineshaft) to generate a more meaningful amount of power (like at least 1 MW). Current grid scale batteries are capable of outputting hundreds of MW of power.

https://en.wikipedia.org/wiki/Battery_storage_power_station

> we can store 3000m^3 of sand, so that's 18MWh.

> I'm sure they've got a lot more space than that, but it just gives some idea of how much sand you're talking about.

They're going to need 3 orders of magnitude more space then because current generation grid scale batteries store GWh of energy, and generally speaking lower cost energy storage competes by offering much higher storage capacity.

I've participated in grid stabilization in Switzerland (ancillary services), and 100kW is absolutely nothing. As far as I recall, 10 years ago, the minimum needed to become a participant was 10MW

Even in the US, I'd bet that few entities bid into the ancillary services markets at only 100kW because the economics don't add up when you are using batteries (due to high fixed costs).

However, having the lower minimum has the advantage of allowing smaller firms to participate (i.e. companies controlling 100s of electric water heaters), instead of just massive utilities.

Yeah, my surface level evaluation of these projects is that they're just not going to be viable - you have the height but you don't have the volume - mineshafts are by definition narrow - and some sort of automated load/unload system is going to run into issues of complexity and reliability and ease of access for maintenance to whatever's at the bottom end.

It's always going to be easier to move water around in an automated fashion, though, so I'm immediately skeptical of any system that isn't some variant of two tanks and a pump/turbine.

If you really do want to use gravity as a power source and don't want to go the hydro route you're better off building narrow-gauge train lines up the sides of hills. The lower-impact and lower-output version of that would be aerial ropeways.

Thinking out loud here, but why sand and not water? Yes, water is only 1000kg/m^3, but probably much easier to transfer around using pumps and pipes. Water can make good use of the horizontal space within the mine, unlike sand. (The graphic in the article shows them using conveyor belts or trucks to move the sand horizontally which seems silly.) Thus the mine could basically be a mini pumped hydro power station, build a new reservoir at the top and use the mine as the bottom reservoir, then pump water water between the two.

When I thought about that silently, my answer was that storing water underground is a challenge when unprotected walls can turn into sludge. Water would require expensive preparation of the existing mine volume, whereas dry material could just be filled in, even stabilising the mine a little in the process. Water and sand sit at different points in the cost per W vs cost per Wh spectrum.

A machine for lifting/lowering loose material is more complicated than a pump, no doubt about that. But a deep shaft would mean that you don't build one machine per shaft, you build a few dozen smaller ones with a good handover mechanism and start getting small serial production benefits right from the first installation. Capacity would be virtually infinite, because with excess energy you could just mine more of whatever stuff is down there.

I guess if it's expensive to switch the dry mass mechanism between directions or speed states, it might be worthwhile to prepare some basin volume up and down and run a small capacity pumped storage in parallel at the same site for higher frequency load changes and short peaks. You might even find yourself discharging the wet battery while charging the dry one or the reverse if there is a sufficient delta in dispatchability.